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prefer-readonly

Require private members to be marked as readonly if they're never modified outside of the constructor.

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

💭

This rule requires type information to run.

Private member variables (whether using the private modifier or private # fields) are never permitted to be modified outside of their declaring class. If that class never modifies their value, they may safely be marked as readonly.

This rule reports on private members are marked as readonly if they're never modified outside of the constructor.

.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/prefer-readonly": "error"
  }
};

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Examples​

class Container {
  // These member variables could be marked as readonly
  private neverModifiedMember = true;
  private onlyModifiedInConstructor: number;
  #neverModifiedPrivateField = 3;

  public constructor(
    onlyModifiedInConstructor: number,
    // Private parameter properties can also be marked as readonly
    private neverModifiedParameter: string,
  ) {
    this.onlyModifiedInConstructor = onlyModifiedInConstructor;
  }
}
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Options​

This rule accepts the following options:

type Options = [
  {
    onlyInlineLambdas?: boolean;
  },
];

const defaultOptions: Options = [{ onlyInlineLambdas: false }];

onlyInlineLambdas​

You may pass "onlyInlineLambdas": true as a rule option within an object to restrict checking only to members immediately assigned a lambda value.

{
  "@typescript-eslint/prefer-readonly": [
    "error",
    { "onlyInlineLambdas": true },
  ],
}

Example of code for the { "onlyInlineLambdas": true } options:

class Container {
  private onClick = () => {
    /* ... */
  };
}
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When Not To Use It​

If you aren't trying to enforce strong immutability guarantees, this rule may be too restrictive for your project.

Type checked lint rules are more powerful than traditional lint rules, but also require configuring type checked linting. See Performance Troubleshooting if you experience performance degredations after enabling type checked rules.

Resources​